Ping-pong turns on the last key, and stepped rows hold the key they last passed (#112)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/core/animation.test.ts

@@ -88,12 +88,37 @@ describe('where the playhead stands', () => {
     expect(playheadAt(10, 10, 'loop', 1.05).frame).toBeCloseTo(0.5);
   });
 
-  it('finishes only in the two once modes, and ping-pong turns at the end', () => {
-    expect(playheadAt(10, 10, 'once', 1.2)).toEqual({ frame: 10, finished: true });
+  it('finishes only in the two once modes, and ping-pong turns on the last key', () => {
+    expect(playheadAt(10, 10, 'once', 1.2)).toEqual({ frame: 10, finished: true, reverse: false });
     expect(playheadAt(10, 10, 'once-reset', 0.5).finished).toBe(false);
-    expect(playheadAt(10, 10, 'pingpong', 1.5).frame).toBeCloseTo(5);
+    expect(playheadAt(10, 10, 'pingpong', 0.95)).toMatchObject({ reverse: true });
+    expect(playheadAt(10, 10, 'pingpong', 0.95).frame).toBeCloseTo(8.5);
+    expect(playheadAt(10, 10, 'pingpong', 1.5).frame).toBeCloseTo(3);
     expect(playheadAt(10, 10, 'loop', 30).finished).toBe(false);
   });
+
+  it('shows each ping-pong frame once per bounce, the two ends included (#112)', () => {
+    const row = keys(...Array.from({ length: 10 }, (_, i): [number, string] => [i, `f${i}`]));
+    const shown: string[] = [];
+    for (let tick = 0; tick < 18 * 6; tick++) {
+      const head = playheadAt(10, 10, 'pingpong', tick / 60);
+      if (tick % 6 === 0) shown.push(trackValueAt(row, 'texture', head.frame, head.reverse) as string);
+    }
+    expect(shown.join(' ')).toBe('f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 f8 f7 f6 f5 f4 f3 f2 f1');
+    expect(trackValueAt(row, 'texture', playheadAt(10, 10, 'pingpong', 1.8).frame)).toBe('f0');
+  });
+
+  it('turns a blended ping-pong value on the last key without standing still', () => {
+    const row = keys([0, 0], [9, 90]);
+    const at = (time: number) => {
+      const head = playheadAt(10, 10, 'pingpong', time);
+      return Number(trackValueAt(row, 'number', head.frame, head.reverse));
+    };
+    expect(at(0.85)).toBeCloseTo(85);
+    expect(at(0.9)).toBeCloseTo(90);
+    expect(at(0.95)).toBeCloseTo(85);
+    expect(at(1.8)).toBeCloseTo(0);
+  });
 });
 
 describe('the value of a row', () => {